Thermal transfer printer, dyesheet and method of operation

ABSTRACT

A thermal transfer printer includes three light emitting diodes ( 7, 8, 9 ) emitting red, green and blue light respectively, and respective detectors ( 10, 11, 12 ) mounted on the opposite side of a dyesheet ( 1 ) passing through the printer. The detectors ( 10, 11, 12 ) detect the light absorption ratios of three colour print panels (Y, M and C) of the dyesheet, and these detected ratios are compared with acceptable ranges of light absorption ratios. If the detected light absorption ratio for any colour falls outside the corresponding range, use or further use of the dyesheet in the printer is prevented, for example by disabling an essential function of the printer or ejecting the dyesheet from the printer.

FIELD OF THE INVENTION

This invention relates to thermal transfer printers, dyesheets thereforand methods of operation thereof.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a thermaltransfer printer including detector means for detecting a lightabsorption characteristic of a thermal transfer dyesheet inserted in theprinter, comparison means for comparing the detected light absorptioncharacteristic with an acceptable light absorption characteristic andrejection means for preventing use or further use of the dyesheet in theprinter if the detected light absorption characteristic fails to conformto the acceptable light absorption characteristic.

The detector means may be operative to detect the light absorptioncharacteristic of one colour only of a multi-colour dyesheet, but toimprove discrimination the detector means is preferably operative todetect the respective light absorption characteristics of more than onecolour, the rejection means then preventing use or further use of thedyesheet if the detected light absorption characteristic of any onecolour fails to conform to the acceptable light absorptioncharacteristic for that colour. For each colour detected, the detectormeans preferably comprises a light source of a frequency appropriate tothe colour to be detected and a detector which produces an electricaloutput signal representative of the attenuation of the light as a resultof passage of the light through the colour of the dyesheet. The lightsource and detector may be on opposite sides of the plane of dyesheetmovement through the printer or may be on the same side, the light thenbeing transmitted a first time through the dyesheet, being reflected andthen being transmitted a second time through the dyesheet. It is alsopossible to obtain further discrimination by measuring the lightabsorption characteristic of a black or overlay panel of the dyesheet.

Preferably, the detected light absorption characteristic is a magnitudeof light absorption and the acceptable light absorption characteristicis a range of light absorption values, the rejection means thenpreventing use or further use of the dyesheet in the printer if thedetected light absorption magnitude falls outside the acceptable range.It is convenient to quantify the absorption magnitude by taking theratio of the detector output with the dye panel in place to the detectoroutput on a clear portion of the dyesheet.

It is also possible for the light absorption characteristic to be themagnitude of optical density, where optical density has its conventionaldefinition of log₁₀ (I₀/I), in which I₀ is the intensity of the incidentlight and I is the intensity of the transmitted light.

The rejection means may operate in any one of a number of ways. Forexample, the rejection means could prevent use or further use of thedyesheet by disabling an essential function of the printer such asdyesheet transport or operation of the print head, or the rejectionmeans could eject the dyesheet from the printer, this being mostpracticable if the dyesheet is carried in a cassette or cartridge. Ineach case, the printer could produce an audible signal and/or a visualindication to the user that the dyesheet is not acceptable.

According to another aspect of the invention there is provided a methodof determining the acceptability of a thermal transfer dyesheet in athermal transfer printer, the method comprising determining a lightabsorption characteristic of the dyesheet, comparing the detected lightabsorption characteristic with an acceptable light absorptioncharacteristic and preventing use or further use of the dyesheet in theprinter if the detected light absorption characteristic fails to conformto the acceptable light absorption characteristic.

The light absorption characteristic may be determined by determining theintensity of light (of a chosen frequency) transmitted by a colour printpanel of the dyesheet.

The dyesheet is normally fed from material wound up on a spool and istaken up after use on a second spool. In order to interrogate thesuccessive panels of a dyesheet, it should desirably be wound past thedetectors. Three possibilities are:

(i) After the dyesheet has been loaded into the printer, part of theinstallation procedure (eg closing the lid of the printer) triggers thedetection process, which is carried out by winding forwards through acomplete sequence, thus wasting one repeat unit of the dyesheet. Thismay not be of great consequence if there are several hundred repeats onthe dyesheet spool. It does, however, mean that only a single check ismade at the beginning, and subsequent panels could be out ofspecification.

(ii) After the dyesheet has been loaded, and at certain other times, thedyesheet is wound forwards to confirm its identity, and then wound backagain, so that none is wasted. This would be a relatively slow processbecause of the need to wind the dyesheet in both directions.

(iii) Printing is carried out as normal, while simultaneously monitoringthe light absorption of the dyesheet. If the dyesheet is inappropriate,the print cycle is aborted. This is potentially the simplest method touse, and in the event that the wrong dyesheet is used would limit wastedmaterial to one unit of dyesheet and receiver.

Instead of interrogating colour print panels of the dyesheet, theintensity of light transmitted through a sample colour area on thedyesheet, corresponding to a colour print panel, may be determined inorder to derive the light absorption characteristic. Preferably, thesesample areas are interrogated by the printer before commencing printing,avoiding any additional winding or rewinding.

According to a yet further aspect of the invention there is provided athermal transfer dyesheet for use in a thermal transfer printer, thedyesheet comprising colour print panels arranged in series along thelength of the dyesheet, the colour print panels being arranged inrepeating groups with each group comprising print panels of threedifferent colours, between each group there being three sample colourareas spaced across the width of the dyesheet and corresponding incolour to the three colours of the print panels. The three differentcolours may be yellow, magenta and cyan, and there may also be aregistration mark between each group of colour print panels.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 shows part of a thermal transfer printer forming one embodimentof the invention,

FIG. 2 shows a part of the length of a dyesheet for use in the printerof FIG. 1,

FIG. 3 is a logic diagram showing operation of the printer of FIG. 1,

FIG. 4 shows part of a thermal transfer printer forming anotherembodiment of the invention,

FIG. 5 shows a part of the length of a dyesheet for use in the printerof FIG. 4, and

FIG. 6 shows an alternative logic diagram.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, the printer has two spaced rollers 2, 3 for guidinga dyesheet 1 in its passage from a supply reel 1 a to a take-up reel 1b. The dyesheet 1 passes between a roller 5 and a thermal print head,not shown. In use, a receiver sheet 4 (e.g. paper or card) is positionedbetween the roller 5 and the dyesheet 1 to receive an image printed onthe sheet 4 by activation of the print head which is in use pressedagainst the dyesheet 1.

The printer also comprises detector means comprising three light sourcesin the form of light emitting diodes 7, 8, 9 emitting red, green andblue light, and respective detectors 10, 11, 12 mounted in a block 6.The light emitting diodes 7, 8, 9 are positioned above the plane oftransport of the dyesheet 1 through the printer, and the detectors 10,11, 12 are positioned below the plane of transport of the dyesheet 1.The three light emitting diodes 7, 8, 9 produce light having respectivewavelengths of 620 an, 525 nm and 430 nm. The light emitting diodes 7,8, 9 are spaced in a direction across the width of the dyesheet 1, andeach source 7, 8, 9 is positioned directly above a correspondingdetector 10, 11, 12.

A representative length of dyesheet 1 is shown in FIG. 2. The dyesheet 1has colour print panels of yellow (Y) magenta (M) and cyan (C) arrangedin series along the length of the dyesheet 1. This group of three colourprint panels repeats along the length of the dyesheet, and between eachgroup there is a transverse registration mark 13 and three sample areasY′, M′ and C′ spaced across the width of the dyesheet and correspondingto the yellow Y, magenta M and cyan C colour print panels. Thus, thereare three sample colour areas which respectively correspond in colourand print density to the yellow magenta and cyan print panels of thedyesheet.

When the dyesheet 1 is located in the printer and transported to theappropriate position, red light from the source 7 passes through thesample area C′ and is detected by the detector 10, so that theelectrical output of the latter is representative of the extent ofattenuation, and therefore light absorption, of the sample area C′ andthus of the print panel C. Similarly, green light from the source passes8 through the sample area M′ and is detected by the detector 11 so thatthe electrical output from the latter is representative of the extent ofattenuation, and therefore light absorption, of the sample area M′ andthus of the panel M. The same considerations apply to the source 9, thedetector 12, the sample area Y′ and the print panel Y. Thus, theelectrical signals from the three detectors 10, 11 and 12 arerepresentative of the light absorption values of the three colour printpanels C, M and Y respectively.

FIG. 3 illustrates how the signals from the detectors 10, 11 and 12 areprocessed in the printer. The magnitude of the signal from the detector12 is used to compute the light absorption ratio of the yellow printpanel Y, as indicated at 14 in FIG. 3. The light absorption ratio is themagnitude of light intensity transmitted through a colour print paneldivided by light intensity transmitted through a clear area of thedyesheet. This light absorption ratio is fed to comparator means whichare pre-programmed with an acceptable range of light absorption ratio,in this case 0.08 to 0.12 and preferably 0.09 to 0.11. In the comparatormeans, the detected light absorption ratio of the yellow print panel Yis compared (as indicated at 15) with the acceptable range. If thedetected light absorption ratio of the yellow print panel Y fallsoutside the acceptable range, the dyesheet is rejected, as indicated at16. If the light absorption ratio of the yellow print panel Y isacceptable, the method proceeds by measuring (at 17) the lightabsorption ratio of the magenta panel M, by reference to the signal fromthe detector 11. In the comparison step 18, the light absorption ratioof the magenta panel M is compared with the acceptable range of 0.04 to0.08, and preferably 0.05 to 0.07. The dyesheet is rejected, asindicated at 19 if the detected light absorption ratio falls outside theacceptable range. If the light absorption ratio of the magenta panel Mis within the acceptable range, the method proceeds (step 20) bymeasuring the light absorption ratio of the cyan panel C, by referenceto the signal from the detector 10. In the comparison step 22 the lightabsorption ratio of the cyan panel C is compared with the acceptablerange of 0.015 to 0.04, preferably 0.022 to 0.034. If the dyesheet failsto conform, it is rejected, step 23. This rejection may involve ejectionfrom the printer of the cassette holding the supply reel 1 a and thetake-up reel 1 b. If the light absorption ratio of the cyan panel C iswithin the acceptable range, the dyesheet is accepted (step 24), havingthen satisfied the criteria for absorption ratios of all three printpanels. Printing by use of the accepted dyesheet can then proceed.

Those skilled in the art will recognise that the absorption ratios atthe absorption maximum translate to higher values at wavelengthsslightly removed from the maximum, and will depend on the broadness ofthe emission band of the light source. It may be desirable to use suchother wavelengths, either because of the availability of a suitablelight source, or in order to reduce the attenuation caused by thedyesheet. The important factor is to match the printer recognitionpattern to the optical properties of dyesheets that are within theacceptable specification. It will also be recognised that, althoughlight emitting diodes provide convenient narrow-band sources, they oftenproduce a further output band in the infrared region of the spectrum.For this reason it is highly desirable to use a detector which isinsensitive to the infrared, as otherwise the discrimination is lost. Itwill also be recognised that it may be convenient to use a singledetector with multiple light sources directed towards it. The sourcescan be switched on in turn in order to provide a sequentialinterrogation of the different colours.

Alternatively, it is possible to employ a broadband light source withmultiple wavelength-selective detectors.

In FIGS. 4 and 5, parts corresponding to those of FIGS. 1 and 2 bear thesame reference numerals. The printer of FIG. 4 differs from the printerof FIG. 1 in that the printer of FIG. 4 is designed to detect lightattenuation through the colour print panels of a dyesheet, not throughsample colour areas. The printer of FIG. 4 has a composite light source25 positioned above the plane of movement of the dyesheet 1 through theprinter, and a single detector 26 positioned below the plane of movementof the dyesheet 1, the detector 26 being aligned with the compositelight source 25 so that the detector 26 detects light from the source 25after attenuation as a consequence of passing through the dyesheet 1.

The composite light source 25 has three individual light sources,respectively producing light having wavelengths of 620 nm, 525 nm and430 nm corresponding to the colours produced by the three diodes 7, 8and 9 of FIG. 1. The detector 26 is sensitive to light at each of thesethree wavelengths. Alternatively, three individual detectors (likedetectors 10, 11 and 12) can be grouped in a single composite detectorpositioned below the plane of movement of the dyesheet 1 through theprinter.

The printer of FIG. 4 assesses the acceptability of a conventionaldyesheet, a portion of which is illustrated in FIG. 5. This dyesheetdiffers from the dyesheet shown in FIG. 2 in that it is devoid of thesample areas Y′, M′ and C′.

When the dyesheet 1 of FIG. 5 is inserted in the printer of FIG. 4, thedyesheet is initially advanced to a first index position at which thefirst yellow colour print panel Y is interposed between the compositesource 25 and the detector 26, so that the electrical signal from thedetector 26 is representative of the light absorption value of theyellow colour print panel Y. The dyesheet 1 is then sequentiallyadvanced to second and third index positions at which the first magentacolour print panel M and the first cyan colour print panels C are inturn interposed between the composite light source 25 and the detector26, so that the detector produces two further electrical signalsrespectively representative of the light absorption values of the colourprint panels M and C.

The three signals from the detector 26 are subjected to processing in alogic sequence corresponding to the flow diagram of FIG. 3. The dyesheet1 of FIG. 5 is thus accepted for printing if the light absorption valuesof all three colour print panels Y, M and C are acceptable. If not, therejection means of the printer are operative to eject the dyesheet.

It will be appreciated that the printer of FIG. 1 could be used toassess the acceptability of a dyesheet 1 of the form shown in FIG. 5,but in this case the signals from the detectors would be produced insuccession as the colour print panels Y., M and C are moved successivelyto their index positions.

EXAMPLE

Light was directed separately from each of three light emitting diodes(LED) towards a silicon photodiode with a built-in infrared cut-offfilter (type VTB8440B, manufactured by EG&G). A voltage of 10.5 V wasapplied to the photodiode, which was connected in series with a 10 M Qresistor. The voltage across the resistor was recorded as a measure ofthe transmitted light intensity. The light absorption ratio wascalculated by taking the ratio of the measured voltage with a panel ofthe corresponding colour in place to the measured voltage with a clearsection of dyesheet in place.

Three different dyesheets were tested in this way:

Nomi- nal Dyesheet 1 Dyesheet 2 Dyesheet 3 Wave- Light Light Lightlength/ Panel Absorption Absorption Absorption LED nm Colour Ratio RatioRatio Kingbright 620 Cyan 0.028 0.0085 0.001 L934SED RS 525 Magen- 0.0610.035 0.002 249-8752 ta Kingbright 430 Yellow 0.098 0.161 0.051 L934MBD

All components were obtained from RS Components Ltd.

The acceptable ranges of light absorption ratios are 0.022 to 0.034 forcyan, 0.05 to 0.07 for magenta and 0.09 to 0.11 for yellow. Dyesheet 1passed on all 3 panels, while dyesheets 2 and 3 failed. This example isapplicable to the printer of FIG. 1 or to the printer of FIG. 4.

FIG. 6 represents a simpler method in which the light absorption of asingle print panel is tested, the result being to accept or reject thedyesheet dependent on whether the detected light absorption is within oroutside the acceptable range of light absorption pre-programmed into theprinter.

The rejection of the dyesheet or ribbon prevents its use or further usein the printer, so the user is obliged to replace the rejected dyesheetor ribbon by a fresh dyesheet or ribbon which is then subjected todetection of its light absorption, as described.

What is claimed is:
 1. A thermal transfer printer including detectormeans for detecting a light absorption characteristic of a thermaltransfer dyesheet inserted in the printer, comparison means forcomparing the detected light absorption characteristic with anacceptable light absorption characteristic and rejection means forpreventing use or further use of the dyesheet in the printer if thedetected light absorption characteristic fails to conform to theacceptable light absorption characteristic.
 2. A thermal transferprinter according to claim 1, wherein the detector means is operative todetect the respective light absorption characteristics of more then onecolour of a multi-colour dyesheet, the rejection means preventing use orfurther use of the dyesheet if the detected light absorptioncharacteristic of any one colour fails to conform to the acceptablelight absorption characteristic for that colour.
 3. A thermal transferprinter according to claim 2, wherein, for each colour detected, thedetector means comprises a light source of a frequency appropriate tothe colour to be detected and a detector which produces an electricaloutput signal representative of the attenuation of the light as a resultof passage of the light through the colour of the dyesheet.
 4. A thermaltransfer printer according to claim 3, wherein the light source anddetector are positioned on mutually opposite sides of the dyesheet.
 5. Athermal transfer printer according to claim 3, wherein the light sourceand detector are on the same side of the dyesheet, the light from thelight source being transmitted through the dyesheet, being reflected andthen being transmitted a second time through the dyesheet beforereaching the detector.
 6. A thermal transfer printer according to anyone of the preceding claims, wherein the detected light absorptioncharacteristic is a magnitude of light absorption.
 7. A thermal transferprinter according to claim 6, wherein the acceptable light absorptioncharacteristic is a range of light absorption values, the rejectionmeans preventing use or further use of the dyesheet in the printer ifthe detected light absorption magnitude falls outside the acceptablerange.
 8. A thermal transfer printer according to any one of claims 1 to5, wherein the detected light absorption characteristic is the magnitudeof optical density, defined as log₁₀ I₀/I where I₀ is the intensity ofthe incident light and I is the intensity of the transmitted light.
 9. Amethod of determining the acceptability of a thermal transfer dyesheetin a thermal transfer printer, comprising determining a light absorptioncharacteristic of the dyesheet, comparing the detected light absorptioncharacteristic with an acceptable light absorption characteristic andpreventing use or further use of the dyesheet in the printer if thedetected light absorption characteristic fails to conform to theacceptable light absorption characteristic.
 10. A method according toclaim 9, wherein the light absorption characteristic is determined bydetecting the intensity of light of a chosen frequency transmitted by acolour print panel of the dyesheet.
 11. A method according to claim 9,wherein the intensity of light transmitted through a sample colour areaon the dyesheet, corresponding to a colour print panel of the dyesheet,is determined in order to derive the light absorption characteristic.12. A method according to any one of claims 9 to 11, wherein the lightabsorption characteristic is a magnitude of light absorption ratio,being the magnitude of light intensity transmitted through a colourprint panel divided by light intensity transmitted through a clear areaof the dyesheet.
 13. A thermal transfer dyesheet for use in a thermaltransfer printer, the dyesheet comprising colour print panels arrangedin series along the length of the dyesheet, the colour print panelsbeing arranged in repeating groups with each group comprising printpanels of three different colours, between each group there being threesample colour areas spaced across the width of the dyesheet andcorresponding in colour to the three colours of the print panels.
 14. Athermal transfer dyesheet according to claim 13 and in combination witha printer according to any one of claims 1 to 8.